The present invention relates to a battery charger, and more particularly, to a battery charger for preventing the charging current from overshooting during the charging mode transition.
In a battery charging system for a lithium-ion (Li-ion) battery, a constant current (CC) mode is adopted to apply a high current to an exhausted battery to activate a rapid charging operation. When the battery is charged to a termination voltage level, the battery charging system switches to a constant voltage (CV) mode to maintain the battery at this desired voltage level. Since there exists an internal resistor in the battery, the battery is not fully-charged at the end of the CC mode. The voltage drop on the internal resistor makes the battery voltage be higher than it really is in the CC mode. After entering the CV mode, the battery voltage will be kept at the desired voltage level. In other words, the battery will be kept charging until the charging current becomes zero. When the charging current is zero, there is no voltage drop on the internal resistor and the battery is fully-charged to the desired voltage level. The CC charging mode cannot be applied to the battery once the battery reaches the desired voltage level because the energy storage capacity of the battery would exceed the nominal rating, leading to AC adaptor, battery and charging system damage. However, the CC mode needs to be used during the first part of the charging operation in order to minimize overall charging time, i.e., the time for charging the battery with the CC mode must be maximized. Therefore, a proper transition between two charging modes is crucial to the battery charging system's performance.
Please refer to FIG. 1, which shows a related art battery charger 100. The related art battery charger 100 is used to charge a battery 150, and includes a charging regulation circuit 110, a comparator 120, a current sensing unit 130, and an operational amplifier 142, wherein the comparator 120 could be a hysteresis comparator for stabling the charging mode. As shown in FIG. 1, it is well-known that the battery 150 is equivalent to a series connection of an internal resistor Rint and an internal capacitor Cint, and the charging regulation circuit 110 is connected to a power supplier (not shown). The comparator 120 is used to compare a battery voltage Vbat of the battery 150 with a reference signal Vref—1 to check whether the battery voltage Vbat is below a threshold. If the battery voltage Vbat is lower than the reference signal Vref—1, the comparator 120 sends out a non-enabling signal D′ to switch on switches SW1, SW3, and to switch off switches SW2, SW4, and then the battery 150 is charged in the CC mode.
In the CC mode the charging regulation circuit 110 is controlled to provide the battery 150 with a constant charging current. As shown in FIG. 1, the charging regulation circuit 110 is configured by a PMOS transistor 111 to regulate the required charging current. The current sensing unit 130 includes a sensor 135, which monitors the charging current flowing through the resistor R. After measuring the charging current flowing through the resistor R, the sensor 135 outputs a voltage V1, which corresponds to the voltage drop across the resistor R, into an operational amplifier 132. For example, if the detected voltage drop is 160 mV, the sensor 135 converts the voltage drop into a voltage level of 160 mV. The operational amplifier 132 sends out a regulation signal S1 to adjust the gate voltage of the PMOS 111 for stabilizing the charging current outputted from the power supplier, which is further explained as follows. Here, the charging regulation circuit 110, the resistor R, the sensor 135, and the operational amplifier 132 form a closed loop. As shown in FIG. 1, the operational amplifier 132 determines the voltage level of the regulation signal S1 by comparing the incoming voltage V1 with a reference signal Vref—2. Assume the charging current during the CC mode is designed to be 10 mA, and the resistance of the resistor R is a known value 50Ω. It is clear that if the charging regulation circuit 110 successfully outputs the desired charging current 10 mA, the voltage drop cross the resistor R will be 500 mV. Therefore, the reference signal Vref—2 is set to 500 mV for checking whether the current flowing through the resistor R has the desired current value. If the voltage V1 is greater than the reference signal Vref—2, the regulation signal S1, which has a higher voltage level amplified by the operational amplifier 132, controls the charging regulation circuit 110 to reduce the charging current; however, if the voltage V1 is less than the reference signal Vref—2, the regulation signal S1, which has a lower voltage level amplified by the operational amplifier 132, controls the charging regulation circuit 110 to increase the charging current. As a result, the battery 150 receives a constant charging current generated from the power supplier.
The battery charger 100 uses the CV mode instead of the CC mode when the battery voltage Vbat is at a termination voltage level, that is, a reference voltage Vref—3. When the battery charger 100 is charging in the CC mode, the comparator 120 keeps comparing the battery voltage Vbat with the reference voltage Vref—3. When the battery voltage Vbat is not less than the reference voltage Vref—3, the comparator 120 sends out an enabling signal D to change the on/off states of the switches. Therefore, switches SW1, SW3 are switched off, and switches SW2, SW4 are switched on. As a result, the battery charger 100 enters the CV mode.
In the CV mode the charging regulation circuit 110 charges the battery 150 to the termination voltage level and the battery charger 100 maintains the battery voltage Vbat at the termination voltage level. In the CV mode, the operational amplifier 142 acts as a regulator to regulate the charging current. The operational amplifier 142 compares the battery voltage Vbat with a reference voltage Vref—3, and sends out a regulation signal S2 to control the gate voltage of the PMOS 111 for further tuning the charging current. Similar to the CC mode, the CV mode also forms a closed loop including the charging regulation circuit 110, the resistor R, and the operational amplifier 142. In order to stabilize the battery voltage Vbat at the reference voltage Vref—3, the operational amplifier 142 compares the reference voltage Vref—3 with the battery voltage Vbat to decide how to regulate the charging current. In other words, the gate voltage of the PMOS transistor 111 is precisely adjusted by the regulation signal S2 when the battery voltage Vbat deviates from the reference voltage Vref—3. As a result, the battery 150 is steadily charged at the constant battery voltage Vbat.
The battery charger 100 will enter the CC mode again when the battery voltage Vbat is lower than the reference voltage Vref—1, for example, the fully-charged battery 150 is removed and a new exhausted battery is connected, which is explained as follows. During charging in the CV mode, the operational amplifier 142 controls the charging regulation circuit 110, while the comparator 120 keeps comparing the battery voltage Vbat with the reference signal Vref—1, which is lower than the reference voltage Vref—3. When a fully-charged battery is taken away and an exhausted battery is connected to the battery charger 100, the battery voltage Vbat becomes low. If the battery voltage Vbat is lower than the reference signal Vref—1, the comparator 120 sends out the non-enabling signal D′ to initialize the battery charger 100, setting it into the CC mode wherein switches SW1, SW3 are on and switches SW2, SW4 are off. Therefore, the battery charger 100 again provides the exhausted battery with a constant charging current.
As mentioned above, there are two modes, the CC mode and the CV mode, selectively used by the battery charger 100; hence there is a transition between these two modes. However, because the comparators 120 and the operational amplifier 142 are not perfectly matched due to well-known manufacturing variations, the transition between these two modes may not be very smooth, and this situation could cause an overshoot charging current to damage an AC adaptor, the battery charger 100 or the battery 150. For example, assume that the constant charging current during the CC mode is 800 mA, the reference signal Vref—1 is 4.1 V, and the reference voltage Vref—3 is 4.2V. Ideally, the transition occurs when the battery voltage Vbat is equal to 4.2V, and then the CV mode closed loop is enabled to maintain the battery voltage Vbat at 4.2V. As a result, the charging current at the transition moment would still be 800 mA, and the transition between two modes would therefore be very smooth. However, practically, because the comparator 120 and the operational amplifier 142 are not matched in their characteristics, the voltage at which the transition occurs and the voltage at which the CV mode loop tries to maintain are likely to be different. That is, if the comparator 120 abnormally sends out the enabling signal D when Vbat is still less than 4.2V, for example 4.1V, and then the operational amplifier 142 will sends out a regulation signal S2 to maintain the battery voltage Vbat at 4.2V by increasing the charging current by controlling the PMOS 111. Therefore, an overshoot charging current capable of damaging the AC adaptor, the battery charger 100 or the battery 150 is likely to be induced. In another case, if the comparator 120 does not send out the enabling signal D when Vbat is already more than 4.2V, for example 4.3V, then the battery 150 is likely to be overcharged which may damage the battery 150.
With this in mind, it is desirable to provide a battery charger which can eliminate both the overshoot charging current and the overcharged battery voltage to prevent the AC adaptor, the battery charger and the battery from being damaged.